1. Field of the Invention
The present invention relates to piezoelectric ceramics and piezoelectric ceramic devices using the same, and more particularly, relates to a piezoelectric ceramic compact as an effective material for forming piezoelectric ceramic devices, such as a piezoelectric ceramic filter, piezoelectric ceramic oscillator and piezoelectric ceramic vibrator, and to a piezoelectric ceramic device using the same.
2. Description of the Related Art
Conventionally, a piezoelectric ceramic compact primarily composed of lead titanate zirconate (Pb(TixZr1xe2x88x92x)O3) or lead titanate (PbTiO3) has been widely used as a piezoelectric ceramic compact used for piezoelectric ceramic devices, such as a piezoelectric ceramic filter, piezoelectric ceramic oscillator and piezoelectric ceramic vibrator. However, since a considerable amount of lead is contained in the piezoelectric ceramic compact primarily composed of lead titanate zirconate or lead titanate, there has been a problem in that the uniformity is degraded due to the evaporation of the lead oxide during the manufacturing process. In order to avoid this problem, a piezoelectric ceramic compact containing no lead or containing a little amount thereof is preferably used.
Since a piezoelectric ceramic compact primarily composed of a bismuth-based layered compound, such as SrBi2Nb2O9, contains no lead oxide, the problem described above does not occur.
In general, a material used for piezoelectric ceramic devices, in particular, for a piezoelectric ceramic oscillator or the like, preferably has a large maximum value (Qmax) of an electrical quality factor Q (1/tan xcex4) in the bandwidth, i.e., in the range from the resonant frequency to the antiresonant frequency. However, a piezoelectric ceramic compact primarily composed of a bismuth-based layered compound, such as SrBi2Nb2O9, has a problem in that a maximum value Qmax which is sufficiently large for practical use cannot be obtained.
Accordingly, an object of the present invention is to provide a piezoelectric ceramic compact as an effective material for forming a piezoelectric ceramic device and the like, which is primarily composed of SrBi2Nb2O9 containing no lead nor lead compounds or containing a little amount thereof, and has a maximum value Qmax improved to a level suitable for practical use. In addition, the present invention provides a piezoelectric ceramic device using the piezoelectric ceramic compact described above.
According to the present invention, in a piezoelectric ceramic compact primarily comprising a bismuth-based layered compound containing Sr, Bi, Nb and oxygen, when the molar ratio of Sr, Bi and Nb contained as primary components in the bismuth-based layered compound is represented by a: b : c, the relationships represented by 0.275xe2x89xa6a/c less than 0.5 and 4xe2x89xa6(2a+3b)/cxe2x89xa64.5 are satisfied.
The piezoelectric ceramic compact according to the present invention may further comprise a divalent metal element other than Sr or a trivalent metal element other than Bi in the range of from more than zero to about 0.1 mol with respect to 1 mol of Nb which is one of the primary components. The divalent metal element other than Sr contained in the piezoelectric ceramic compact is preferably at least one element selected from the group consisting of Mg, Ca, Ba and Pb. In addition, the trivalent metal element other than Bi contained in the piezoelectric ceramic compact is preferably at least one element selected from the group consisting of Sc, Y, La, Ce, Pr, Nd, Sm, Gd, Dy, Er and Yb.
The piezoelectric ceramic compact according to the present invention may further comprise Ta in the range of from more than zero to less than about 0.25 mol with respect to 1 mol of Nb of the primary components.
In addition, the piezoelectric ceramic compact according to the present invention may further comprise Mn in the range of from more than zero to about 1.5 percent by weight as MnCO3.
A piezoelectric ceramic device according to the present invention comprises the piezoelectric ceramic compact of the present invention and electrodes provided for the piezoelectric ceramic compact.
In this connection, the piezoelectric ceramic compact of the present invention is a sintered material formed by firing a starting composition for the piezoelectric ceramic compact; however, hardly any change in the composition ratio described above is observed before and after firing.
According to the present invention, in the piezoelectric ceramic compact primarily comprising a bismuth-based layered compound containing Sr, Bi, Nb, and oxygen, when the molar ratio of Sr, Bi and Nb contained as primary components in the bismuth-based layered compound is represented by a: b: c, the relationships of the molar ratio are determined such that 0.275xe2x89xa6a/c less than 0.5 and 4xe2x89xa6(2a+3b)/cxe2x89xa64.5. The reason for this is that Qmax at a level suitable for practical use cannot be obtained when the composition is outside the relationships described above.
In addition, when the piezoelectric ceramic compact of the present invention further comprises a divalent metal element other than Sr or a trivalent metal element other than Bi in the range of from more than zero to about 0.1 mol with respect to 1 mol of Nb of the primary components, the advantage of the present invention becomes more significant. The reason the piezoelectric ceramic compact comprises a divalent metal element other than Sr or a trivalent metal element other than Bi in the range of from more than zero to about 0.1 mol with respect to 1 mol of Nb is that when the content thereof exceeds the range described above, the Qmax tends to decrease compared to the case in which the metal element described above is not present.
In addition, when the piezoelectric ceramic compact of the present invention further comprises Ta in the range of from more than zero to less than about 0.25 mol with respect to 1 mol of Nb of the primary component, the advantage of the present invention becomes more significant. The reason the piezoelectric ceramic compact comprises Ta in the range of from more than zero to less than about 0.25 mol with respect to 1 mol of Nb is that when the content thereof exceeds the range described above, the Qmax tends to decrease compared to the case in which Ta is not present.
Furthermore, when the piezoelectric ceramic compact of the present invention further comprises Mn in the range of from more than zero to about 1.5 percent by weight as MnCO3, the advantage of the present invention becomes more significant. The reason the piezoelectric ceramic compact comprises Mn in the range of from more than zero to about 1.5 percent by weight as MnCO3 is that when the content thereof exceeds the range described above, the Qmax tends to decrease compared to the case in which Mn is not present.
The objects described above, further objects, features, and advantages of the present invention will become more apparent from the following detailed description of the embodiments.